Methanol is used as a feed stock for a variety of chemical manufacturing processes. One process that is more recently being developed is the conversion of methanol to olefin products, particularly products containing the olefins ethylene and propylene. The olefins produced from the methanol conversion process are of suitable quality to be used in polymer manufacturing processes. Of a commercial concern in the methanol conversion process, however, is whether sufficient quantities of light olefins (i.e., ethylene and propylene) can be produced. Another concern is whether an appropriate catalyst can be supplied at sufficient quantities to meet the rigors of commercial scale processing.
Conventional molecular sieves used in converting oxygenates to olefins are zeolites and various metalloaluminophosphates. For example, U.S. Pat. No. 5,367,100 describes the use of the zeolite, ZSM-5, to convert methanol into olefin(s); U.S. Pat. No. 4,062,905 discusses the conversion of methanol and other oxygenates to ethylene and propylene using crystalline aluminosilicate zeolites, for example Zeolite T, ZK5, erionite and chabazite; U.S. Pat. No. 4,079,095 describes the use of ZSM-34 to convert methanol to hydrocarbon products such as ethylene and propylene; and U.S. Pat. No. 4,310,440 describes producing light olefin(s) from an alcohol using a crystalline aluminophosphate, often designated AlPO4.
Some of the most useful molecular sieves for converting methanol to olefin(s) are silicoaluminophosphate molecular sieves. Silicoaluminophosphate (SAPO) molecular sieves contain a three-dimensional microporous crystalline framework structure of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units. SAPO synthesis is described in U.S. Pat. No. 4,440,871, which is herein fully incorporated by reference. SAPO molecular sieves are generally synthesized by the hydrothermal crystallization of a reaction mixture of silicon-, aluminum- and phosphorus-sources and at least one templating agent. Synthesis of a SAPO molecular sieve, its formulation into a SAPO catalyst, and its use in converting a hydrocarbon feedstock into olefin(s), particularly where the feedstock is methanol, are disclosed in U.S. Pat. Nos. 4,499,327, 4,677,242, 4,677,243, 4,873,390, 5,095,163, 5,714,662 and 6,166,282, all of which are herein fully incorporated by reference.
Typically, molecular sieves are formed into molecular sieve catalyst compositions (generally referred to as formulated catalysts) to improve their durability in commercial conversion processes. These formulated catalyst compositions are conventionally formed by combining molecular sieve, and one or more matrix materials, with a binder. The binder acts to hold the matrix material to the molecular sieve.
U.S. Pat. No. 4,465,889 describes a catalyst composition comprising a silicalite molecular sieve impregnated with a thorium, zirconium, or titanium active metal oxide for use in converting methanol, dimethyl ether, or a mixture thereof into a hydrocarbon product rich in iso-C4 compounds.
U.S. Pat. No. 6,180,828 discusses the use of a modified molecular sieve to produce methylamines from methanol and ammonia, where for example, a silicoaluminophosphate molecular sieve is combined with one or more modifiers, such as a zirconium oxide, a titanium oxide, an yttrium oxide, montmorillonite or kaolinite.
U.S. Pat. No. 5,417,949 relates to a process for converting noxious nitrogen oxides in an oxygen containing effluent into nitrogen and water using a molecular sieve and a active metal oxide binder, where the preferred binder is titania and the molecular sieve is an aluminosilicate.
EP-A-312981 discloses a process for cracking vanadium-containing hydrocarbon feed streams using a catalyst composition comprising a physical mixture of a zeolite embedded in an inorganic refractory matrix material and at least one oxide of beryllium, magnesium, calcium, strontium, barium or lanthanum, preferably magnesium oxide, on a silica-containing support material.
Kang and Inui, “Effects of decrease in number of acid sites located on the external surface of Ni-SAPO-34 crystalline catalyst by the mechanochemical method,” Catalysis Letters 53, pages 171–176 (1998) disclose that the shape selectivity can be enhanced and the coke formation mitigated in the conversion of methanol to ethylene over Ni-SAPO-34 by milling the catalyst with MgO, CaO, BaO or Cs2O on microspherical non-porous silica, with BaO being the most preferred.
International Publication No. WO 98/29370 discloses the conversion of oxygenates to olefins over a small pore non-zeolitic molecular sieve containing a metal selected from the group consisting of a lanthanide, an actinide, scandium, yttrium, a Group 4 metal, a Group 5 metal or combinations thereof.
U.S. Pat. No. 4,677,242 (Kaiser) describes the use of a silicoaluminophosphate (SAPO) molecular sieve catalyst for converting various oxygenates, such as methanol, to olefins. According to the patent, the SAPO catalyst is an extremely efficient catalyst for the conversion of oxygenates to light olefin products when the feed is converted in the presence of a diluent. The diluent used has an average kinetic diameter larger than the pores of the SAPO molecular sieve. The selected SAPO molecular sieves have pore sizes capable of absorbing oxygen (average kinetic diameter of about 3.36 angstroms), but with negligible adsorption of isobutane (average kinetic diameter of about 5.0 angstroms).
U.S. Pat. No. 6,046,372 (Brown et al.) discloses another method of converting methanol to light olefins. The method incorporates the use of medium pore zeolite molecular sieves, particularly medium pore ZSM type zeolites, in converting methanol and/or dimethyl ether to light olefin. Light olefin production is aided by the use of an aromatic compound as a co-feed. The aromatic compound has a critical diameter less than the pore size of the catalyst, and is capable of alkylation by the methanol and/or dimethyl ether. Ethylene product selectivity is believed to be derived from the back-cracking of ethyl-aromatic intermediates. The formation of the ethyl-aromatic intermediates is believed to be facilitated by a mechanism in which the aromatic compound effectively acts as a catalyst in the conversion of two molecules of methanol to one molecule of ethylene.
U.S. Pat. No. 6,051,746 (Sun et al.) also describes a method for increasing light olefin selectivity in the conversion of oxygenates using a small pore molecular sieve catalyst. The selectivity is increased by exposing a catalyst to a modifier before or during the conversion reaction. The modifier is a polynuclear aromatic having at least three interconnected ring structures, with each ring structure having at least 5 ring members. It is adsorbed onto the catalyst prior to or simultaneously with the introduction of feed.
U.S. Pat. No. 6,137,022 (Kuechler et al.) is to a process for increasing the selectivity of a reaction to convert oxygenates to olefins. The process involves contacting the oxygenate in a reaction zone containing 15 volume percent or less of a catalyst comprising SAPO molecular sieve, and maintaining conversion of the feedstock between 80% and 99% under conditions effective to convert 100% of the feedstock when the reaction zone contains at least 33 volume percent of the molecular sieve material. The process is considered to be beneficial in maximizing the production of ethylene and/or propylene, and to minimize the production of undesired products.
U.S. Pat. No. 6,225,254 (Janssen et al.) is directed to a method of maintaining acid catalyst sites of a SAPO molecular sieve catalyst. According to the patent, catalyst sites are lost when exposed to a moisture-containing environment. In order to maintain the catalyst sites, and thereby preserve catalyst activity, template-containing SAPO molecular sieves are heated in an oxygen depleted environment under conditions effective to provide an integrated catalyst life that is greater than that obtained in a non-oxygen depleted environment.
U.S. Pat. No. 6,436,869 (Searle et al.) is directed to a method of obtaining olefin product high in ethylene and/or propylene content, while reducing the amount of any one or more of C1–C4 paraffin by-products, and to reduce the amount of coke deposits on the catalyst during the reaction. The method is accomplished by providing a catalyst that comprises SAPO crystals, a binder comprising AlPO crystals, and nickel, cobalt and/or iron, wherein the catalyst does not contain significant amounts of amorphous binder, but rather contains crystalline AlPO.
U.S. Pat. No. 6,437,208 (Kuechler et al.) discloses a method for making olefin product from an oxygenate-containing feedstock. In the method, a SAPO molecular sieve catalyst is contacted with the oxygenate-containing feedstock in a reactor at an average catalyst feedstock exposure index of at least 1.0. The average catalyst feedstock exposure index is the total weight of oxygenate plus hydrocarbon fed to the reactor divided by the total weight of fresh and regenerated SAPO molecular sieve (i.e., excluding binder, inerts, etc., of the catalyst composition) sent to the reactor, both total weights measured over the same period of time. The method is shown to be effective in maintaining a high ethylene and propylene selectivity.
WO 01/62382 A2 (ExxonMobil Chemical Patents Inc.) discloses that selectivity to ethylene and propylene can be increased by pretreating a SAPO molecular sieve to form an integrated hydrocarbon co-catalyst within the framework of the molecular sieve prior to contacting with oxygenate feed. Acetone, methanol, propene, butene, pentene and hexene are given as examples of pretreatment compounds capable of forming an integrated hydrocarbon co-catalyst. The conditions for pretreatment include pretreating at a lower temperature relative to the reaction temperature. A preferred pretreatment vessel is an auxiliary fluidized bed reactor system associated with the oxygenate conversion reactor.
U.S. Patent Application, Publication No. US 2003/0176753 A1 (Levin et al.), discloses a catalyst composition comprising a molecular sieve and at least one oxide of a metal selected from Group 3 of the Periodic Table of Elements, the Lanthanide series of elements and the Actinide series of elements. The metal oxide has an uptake of carbon dioxide at 100° C. of at least 0.03, and typically at least 0.04, mg/m2 of the metal oxide. The catalyst is useful in converting oxygenate compounds into one or more olefins, preferably ethylene and/or propylene.
U.S. Patent Application, Publication No. US 2003/0176752 A1 (Levin et al.), discloses another metal oxide catalyst composition useful in converting oxygenate compounds into one or more olefins, preferably ethylene and/or propylene. The catalyst composition comprises a molecular sieve and at least one oxide of a metal selected from Group 4 of the Periodic Table of Elements. The metal oxide has an uptake of carbon dioxide at 100° C. of at least 0.03, and typically at least 0.035, mg/m2 of the metal oxide.
U.S. Patent Application, Publication No. US 2003/0176733 A1 (Xu et al.), discloses another type of metal oxide catalyst composition useful in converting oxygenate compounds into one or more olefins, preferably ethylene and/or propylene. The catalyst composition comprises a silicoaluminophosphate molecular sieve and a metal oxide which has a surface area greater than 20 m2/g, which has been calcined at temperature greater than 200° C. When saturated with acetone, and contacted with acetone for 1 hour at 25° C., the catalyst converts more than 80% of the acetone. The molecular sieve has an average pore size of less than 5 angstroms.
In spite of the recent technological advances in converting oxygenates to olefins, there remains a need to further increase the quantity of light olefins in the conversion product. In particular, there remains a need to increase product selectivity to ethylene and propylene, and particularly to ethylene. There also remains a need to reduce the amount of undesirable by-products in converting the oxygenates to olefins. Additionally, there remains a need to provide catalysts that have characteristics that enable the catalysts to endure the various rigors of commercial demands. Catalysts that have substantially increased catalyst lifetimes are also of value.